This invention relates generally to the detection of excessive negative offset shift of a sensor signal, particularly, a signal conditioned sensor signal.
Systems interpreting sensor outputs, herein simply called System, often can not detect if a sensor""s output offset (i.e., sensor output at minimum stimulus input) has shifted negatively more than an acceptable amount due to one or both of the following:
1. The shifted sensor""s output may be outside the System""s input range, or
2. The sensor may clamp outputs below a certain level thereby masking the actual negative offset shift.
An object of the invention is the provision of a sensor which overcomes the above noted prior art limitations. Another object is the provision of a method and apparatus for sensitively detecting negative offset shifts and providing a sensor output by the System indicative of a sensor fault. Yet another object is the provision of such a method and apparatus which provide detection of offset shifts due either to drift in the sensor per se or in the compensation of associated circuitry. Another object is the provision of a method and apparatus for sensitively detecting a negative offset shift of a piezoresistive Wheatstone sensor output and providing a sensor output by the System indicative of a fault.
Briefly in accordance with the invention, the offset compensated signal within a signal conditioning circuit is compared to a reference voltage based on an unobtainable stimulus input condition to determine a fault status. In the event that the fault status is set, the output is driven to an output level interpreted by the System as a fault. The sensor electronic elements include a sense element coupled to an ASIC (application specific integrated circuit) having signal conditioning circuitry including offset and gain compensation.
Although it is not necessary to compensate gain prior to the fault status comparison, superior negative offset drift detection sensitivity can be achieved by basing the comparison on a both offset and gain compensated signal due to ASIC tolerances (i.e., changes in ASIC gain may influence offset).